This application claims the benefit of U.S. Provisional Application No. 60/247,198, which was filed on Nov. 13, 2000.
The present invention relates generally to filter media, and more particularly to a method for treating polymeric fibers for use as filter media in the removal of particulates from a gas stream.
As used herein, the term baghouse refers to any of a number of dry filtering devices which operate by passing particulate-laden air or gas through bag-shaped filters to rid the air or gas of the particulate. Baghouses are usually operated at high flow rates or pressures, and the filter bags, which may be formed of various types of filtering media, operate by trapping or collecting the particulate material as it comes in contact with the bag.
As used herein, the term particulate material refers to dust and other particles which are suspended by or entrained in the air or gas which is being passed through the filter or through the baghouse.
As used herein, the terms filter, filter material and filter media refer to a mat of fibers, fabric or other material which is permeable to air or other gases and which is capable of filtering particulate material from the air or gas that is passed through the material.
As used herein, the term mat of fibers refers to a plurality of fibers which are woven, felted, needlepunched or otherwise intertwined in such a manner as to form a unitary structure.
As used herein, the term filtration surface refers to the surface or side of a filter or filter bag which is the first to contact the air or gas which is being filtered thereby.
As used herein, the term pores refers to the spaces or interstices between the fibers which are created by the intertwining of fibers in a mat. The pores may vary in size and shape, and a number of intercommunicating or interconnecting pores may provide one or more passageways for air and gases through the mat.
As used herein, the term average pore size at the filtration surface means the average of the area of the pores on a filtration surface of a mat of fibers. The average pore size at the filtration surface may differ from the average of the pore size within the mat.
As used herein, the terms permeability and flow rate refer to the rate at which a volume of air or other gas may be passed through a filter, per unit of time. The initial flow rate or the initial permeability of a filter is that which exists (and may be measured) prior to the filter""s exposure to entrained particulate material in the normal filtering environment. The operating flow rate or the operating permeability of a filter is that which exists (and may be measured) during or after the filter""s exposure to entrained particulate material in the normal filtering environment.
As used herein, the term polyimide fibers refers to filaments and fibers comprised of any of a number of flame-retardant, high-temperature resistant compounds of the general formula: 
where n is an integer larger than 1, and
A is a tetravalent aromatic group selected from: 
and wherein X represents CO, CH2, O, S or CF2, and R is at least one of the following divalent aromatic groups: 
As used herein, the term nip rolls refers to at least a pair of rollers having axes that are positioned in such proximity to one another that the outer surfaces of the rollers are close enough to each other to apply pressure to a material which is being passed between them. The axes about which the nip rolls rotate may be capable of being adjusted so that the pressure applied by the nip rolls may be increased or decreased.
There are many known applications requiring the filtration of particulate impurities from air or other gases which are derived from industrial processes. A common method for filtering particulate materials from gases involves the use of woven or felted bag filters that are mounted inside a metal structure commonly referred to as a baghouse. In most baghouses, a number of filters are provided, each in the form of a generally cylindrical bag that is closed at one end and fitted over a metal frame. The flow of air or gas within the baghouse is directed from the outside of each bag through the fabric wall of the bag. Consequently, particulate material entrained in the air or gas passing into the baghouse will collect on the exterior filtration surface of each of the bags while the filtered air or gas passes through the fabric wall and into the interior of each bag, and then to a plenum located generally above the bags.
Periodically, a blast of high pressure air may be sent through each bag in a direction opposite to the normal flow, i.e. from inside of the bag out through the bag fabric. Generally, such a high pressure blast is strong enough to dislodge most of the dust or other particles that have collected on the outer (filtration) surface of the bag fabric, although it may not dislodge the dust that has penetrated into the internal pores of the fabric. The dust particles that are dislodged by the air blast will fall to the bottom of the baghouse where they are collected and discharged, most commonly by way of a screw conveyor.
While a number of filter fibers and fabrics are known for use in baghouses, many of these materials are unsuitable for high-temperature applications. More specifically, many of the known filter fabrics are not capable of continuous operation at temperatures at or above approximately 400xc2x0 F. However, baghouses in many common applications require filtration of hot gases, and often, these gases are introduced to the baghouse at a temperature of 400xc2x0 F. or higher. One type of material that may be utilized in the manufacture of filters for baghouses where high temperature gases will be encountered is polyimide fiber, which is known to be resistant to high temperatures. A fabric of an aromatic polyimide fiber that is sold for use in filter bags is marketed under the trademark P84 by Lenzing Fibers Corporation of Lowland, Tenn.
Materials made of polyimide fibers are commonly used as insulating and fire extinguishing blankets, and in other products where heat resistance is importance. U.S. Pat. No. 5,384,390 of Schobesberger et al. discloses a composition for heat-resistant, flame-retardant polyimide fibers, and U.S. Pat. No. 5,271,889 of Weinrotter et al. discloses a use of such fibers in the production of molded products. U.S. Pat. No. 5,181,945 of Bodovsky teaches that polyimide fibers can be knitted into a backing, and loose fibers of the same material can be mechanically interlocked into the knitted backing to form a tufted pile. The resulting fur-like fabric may then be used as an air filter in a rotating drum filtering machine.
The selection of a material with suitable thermal properties for baghouse filter applications is not the only problem faced by baghouse filter designers. Most of the filters used in such applications are comprised of fabrics which filter particulate materials from a gas stream by trapping such particulates within the pores between the fibers of the fabric. This results in clogged or blocked pores in and through the fabric and ultimately in a lower air or gas flow rate through the filter bag. Due to the clogging or blocking of the pores in the fabric of conventional filter bags by particulate matter (and the resultant reduced flow rate of air or gas through the bag), it is frequently necessary to use numerous bags and large baghouses in order to achieve the desired filtering efficiency as well as to maintain the required flow of air through the baghouse.
Although a conventional filter may be made of materials having very closely spaced fibers so as to produce a small average pore size within and through the filter in order to increase the amount of dirt or other particulates that may be captured by the filter, the small pore sizes of such filter will also decrease the permeability or the flow rate of air or gas through the filter. This decrease in the flow rate through the filter reduces its efficiency. Furthermore, as the filter begins to accumulate dust in its pores in use, the pores in and through the filter become blocked with collected particulate so that the operating flow rate of air or gas through the filter is further decreased. Therefore, large bags and/or a large number of bags are conventionally required within a baghouse to insure proper flow and cleaning efficiency. The size and/or number of bags which are required is determinative of the size of the baghouse and of the fans which are needed to pass the air into the baghouse and through the filter bags.
It would be desirable if the efficiency of a filter could be increased while minimizing the decrease in the operating flow rate of such filter during use. It would also be desirable if the number and size of the filter bags could be decreased, along with the size of the baghouses required for common filtering applications.
Accordingly, it is an advantage of the present invention to provide an improved filter for use in baghouses or the like, which is treated so as to collect a larger portion of the entrained particulate at the filtering surface. It is another advantage of the present invention to provide an improved filter having an operating flow rate that is closer to its initial flow rate than would be expected in a conventional filter. It is yet another advantage of the present invention to provide a filter which is more efficient than conventional filters and which thereby allows the use of fewer and smaller bags within the baghouse, which in turn allows for a reduction in the size of a baghouse and in the capacity of the fans needed to operate it. Another advantage is that the increased efficiency imparted to filters treated or constructed according to the invention may permit an increase in the rate of flow of dust-laden air into the baghouse. An advantage of a preferred embodiment of the invention is the provision of a laminated filter fabric in which the preferred polyimide material treated according to the invention may be combined with a conventional felt fabric or other known filtering material, supporting substrate or scrim, thereby providing a high-efficiency filter of reduced weight and cost.
Other objects, advantages and features of the present invention will become apparent to those skilled in the art to which the invention relates from an examination of the drawings and the ensuing description.
The present invention provides a method for making an improved filter for use in dry filtering devices such as baghouses and the like. A mat of polymeric fibers is provided, and a filtration surface of the mat is treated by applying pressure thereto while heating the surface to a temperature sufficient to soften and compact the fibers on the surface of the mat so as to reduce the average pore size at the filtration surface. The treated mat may then be formed into the shape of a filter for use in filtering air and gases, with the treated surface being disposed so as to be the first surface to contact the air or gas which is being filtered thereby.
The reduction in the pore size at the filtration surface of the mat will prevent a significant portion of entrained particulate from penetrating beyond the outer (filtration) surface of the filter mat. Such particles will be retained on or at the filtration surface, and consequently, will not become embedded within the mat. The particulate that is retained on the filtration surface of the filter will not block or clog the internal pores of the mat, and consequently, the improved filter of the invention will exhibit a higher operating flow rate or permeability for gas or air flow when particulate is trapped thereon than will a conventional filter. Furthermore, because the improved filter of the invention tends to retain much of the particulate on or at the surface, these particles may be more readily removed from the mat with a pulse of air in the reverse-flow direction.
Due to the fact that penetration of particulates beyond the filtration surface is significantly reduced in the invention, the thickness of filter media needed for a particular filtering application may be reduced. This allows for a reduction in the thickness and weight of filters made according to the invention, when compared to conventional bags of similar cleaning efficiency. Because of the increased efficiency imparted to filters according to the invention, the filtration surface area, thickness and weight of the filter bags, the number of filter bags in a baghouse and the size of the baghouse itself may be reduced from that required for comparable filtration using conventional filter bags. The invention also contemplates a use of the improved filter media as a laminate that is applied to a conventional filter material, scrim or other appropriate substrate.
In order to facilitate an understanding of the invention, the preferred embodiments of the invention are illustrated in the drawings, and a detailed description thereof follows. It is not intended, however, that the invention be limited to the particular embodiments described or to use in connection with the apparatus illustrated herein. Various modifications and alternative embodiments such as would ordinarily occur to one skilled in the art to which the invention relates are also contemplated and included within the scope of the invention described and claimed herein.